Preparation of porous structures

ABSTRACT

METHOD OF PREPARING A POROUS POLYMER WHICH COMPRISES POLYMERIZING EITHER AN ALKENYL AROMATIC MONOMER, AN ACRYLATE MONOMER, A VINYL ESTER MONOMER, OR MIXTURES THEREOF AND POLYMERIZABLE CROSS-LINKING AGENT IN A BINARY SOLVENT HAVING A CERTAIN RANGE OF SOLUBILITY PARAMETER.

Jan. 12, 1971 T. ALFREY. JR..

PHEPARATION OF POROUS STRUCTURES Original Filed July 1. 1966 I N VENTORS. Turn er fl/f'r' Will/hm 6. L

United States Patent Office Re. 27,026 Reissued Jan. 12, 1971 27,026PREPARATION OF POROUS STRUCTURES Turner Alfrey, Jr., Midland, Mich., andWilliam G. Lloyd, Bowling Green, Ky., assignors to The Dow ChemicalCompany, Midland, Mich., a corporation of Delaware Original No.3,322,695, dated May 30, 1967, Ser. No. 563,634, July 1, 1966, which isa continuation-in-part of Ser. No. 253,084, Jan. 22, 1963. Applicationfor reissue Mar. 20, 1969, Ser. No. 817,231

Int. Cl. C08f 47/08; C08j 1/14 US. Cl. 2602.5 6 Claims Matter enclosedin heavy brackets II] appears in the original patent but forms no partof this reissue specification; matter printed in italics indicates theadditions made by reissue.

ABSTRACT OF THE DISCLOSURE Method of preparing a porous polymer whichcomprises polymerizing either an alkenyl aromatic monomer, an acrylatemonomer, a vinyl ester monomer, or mixtures thereof and a polymerizablecross-linking agent in a binary solvent having a certain range ofsolubility parameter.

This application is a continuation-in-part of copending application Ser.No. 253,084 filed Jan. 22, 1963 now abandoned.

This invention relates to the preparation of porous structures. It moreparticularly relates to a method of preparing microporous syntheticresinous materials.

A wide variety of cellular and permeable polymeric materials are knownwhich are formed oftentimes by polymerization into a generally porousstructure or frequently are prepared by incorporating within a resinousmaterial either after or during its formation an agent which willgenerate a gas and cause a plurality of interconnecting pores orpassages within the polymeric body. Other methods are known whichinclude polymerizing a resinous material in the presence of aparticulate solid which may at a later time if desired be dissolved,leaving a resinous structure with interconnecting pores. Suchparticulate material may be incorporated into thermoplastic bodies bymilling and similar mixing procedures and the porous body subsequentlygenerated by dissolving away the solid material. By employing certainliquids which are incompatible or non-solvents for the polymericmaterial, oftentimes the porous bodies can be generated by polymerizingin the presence of such liquids.

The foregoing methods do not provide a means of controlling the poresize of the porous particles, particularly within a high range of poresizes below about it] microns. Such resinous materials having extremelysmall pore size, that is, having pores ranging in size from about 20Angstroms to about 1 micron in diameter are particularly beneficial andadvantageous for use in the separation of solutions into their variouscomponents by selective absorption.

It is an object of this invention to provide a method of controlling thepore size of synthetic resinous materials during their polymerization.

It is a further object of this invention to provide a method ofpolymerizing a monomeric material into a resinous material having anaverage predetermined pore diameter.

It is a further object of this invention to utilize selective solventsfor preparing polymer bodies of predetermined porosity.

These benefits and other advantages in accordance with the invention arereadily achieved by polymerizing an organic material in admixture withfrom about /2 to about 20 times the weight of the material of a solventwhich is miscible with the unpolymerized material and exhibits limitedsolubility for the polymeric form of the material, thereby forming arigid cross-linked polymeric body having a plurality of interconnectingpores therein.

The structure of the polymers in accordance with the invention may bereadily understood by reference to the drawing wherein:

The figure depicts an enlarged view of a portion of a polymer bodyprepared in accordance with the invention wherein the referencecharacter A refers to the portions of the polymer body and B designatesthe voids or spaces therebetween.

A wide variety of monofunctional olefinically unsaturated polymerizablematerials, including monovinyl materials may be employed in the practiceof the present invention. Particularly advantageous are the alkenylaromatic monomers. By the term alkenyl aromatic is meant an alkenylaromatic compound having the general formula:

Ar CZCIIZ wherein Ar represents an aromatic hydrocarbon radical, or anaromatic halohydrocarbon radical of the benzene series, and R ishydrogen or the methyl radical. Examples of such alkenyl aromaticmonomers are styrene, a-methylstyrene, ortho-methylstyrene,meta-methylstyrene, paramethylstyrene, ar-ethylstyrene, ar-vinylxylene,ar-chlorostyrene, or ar-bromostyrene and the like.

The acrylate monomers alone or in combination with the alkenyl aromaticmonomers may also be utilized. Such acrylate-type monomers includemonomers of the formula:

wherein R is selected from the group consisting of hydrogen and an alkylradical containing from about 1 to 12 carbon atoms and R is selectedfrom the group consisting of hydrogen and methyl. Typical acrylatematerials which may be used are methyl methacrylate, ethyl acrylate,propyl acrylate, butyl acrylate, butyl methacrylate, propylmethacrylate, lauryl acrylate, Z-ethylhexylacrylate, ethyl methacrylateand the like. Copolymers of vinyl chloride and vinylidene chloride,acrylonitrile with vinyl chloride, vinyl bromide, and similarhalogenated vinyl compounds may be prepared by the process of theinvention. Esters, such as vinyl esters having the formula:

0 l CIIFTCH O l. -R

wherein R is an alkyl radical containing from 2 to 18 carbon atoms, mayalso frequently be employed with benefit. Typical monomers fallingwithin this classification are vinyl acetate, vinyl butyrate, vinylstearate, vinyl laurate, vinyl myristate, vinyl propionate, and thelike.

Typical copolymerizable acids are acrylic acid, methacrylic acid,itaconic acid, citraconic acid, maleic acid, fumaric acid, oleic acid,vinylbenzoic acid, and the like.

Advantageously the synthetic polyester resins which are prepared byreacting terephthalic acid and dialkyl terephthalics or ester-formingderivatives thereof, with a glycol of the series HO(CH ),,OH, where n isa whole number within the range of 2-10 and having reactive olefiniclinkages within the polymer molecule, are readily utilized in thepractice of the invention. Such polyesters also may includecopolymerized therein up to 20 percent by weight of a second acid orester thereof having reactive olefinic unsaturation such as fumaricacid, maleic acid, itaconic acid, tetrahydrophthalic acid, and the like.Polyesters, containing olefinic unsaturation are readily reacted withmonomeric olefinic materials such as the alkenyl aromatic monomers ofthe general formula:

wherein Ar represents an aromatic hydrocarbon radical, or aromatichalohydrocarbon radical of the benzene series, and R is hydrogen or themethyl radical. Examples of alkenyl aromatic monomers are styrene,a-methylstyrene, ortho-methylstyrene, meta-methylstyrene,paramethylstyrene, ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene,and ar-bromostyrene; beneficially, if desired, other olefinicallyunsaturated polymerizable monomeric materials may be utilized such asacrylates and methacrylates, acrylonitrile, divinylbenzene, vinylacetate, vinyl butyrate, and the like.

A number of polyfunctional olefinically unsaturated or difunctionalmonomeric constituents are readily employed in the invention.Advantageously, with the vinyl or alkenyl aromatic type monomers,suitable difunctional cross linking agents as divinylbenzene, diethyleneglycol dimethacrylate, diallyl fumarate, diallyl phthalate, and the likeare readily utilized.

The porous polymers prepared according to the method of the presentinvention are preferably prepared utilizing at least mole percent of across linking agent when vinyl or vinylidene type monomers are used. Itis essential that the desired product be provided with sufficient crosslinking in the polymer to prevent significant swelling thereof in thepresence of solvent. Such swelling closes or reduces the dimensions ofthe pores in many cases to the point where the product loses itsdesirable characteristics. Further any polymer which shows insufficientcross linking is oftentimes sufficiently soft that it will pack, deformand if being used for filtration or similar purposes, will transmitlittle or none of the material which is being passed therethrough.

Accordingly, the choice of solvents is equally wide depending upon theparticular monomer system that is utilized for the production of theporous polymeric material. The solvent should not be such a good solventfor the polymer that it is completely miscible in all proportions, norshould the solvent be a material which is a non-solvent for the monomer.Generally materials which are completely soluble result in a polymerproduct which has no apparent pore size and is simply swollen by thesolvent whereas polymeric materials produced by the use of non-solventsresult in a pore size that is much too large for such operations as theselective absorption and the like. Suitable solvents for the practice ofthe invention are readily prepared by admixing the solvents andnon-solvents or alternately by selecting a suitbale solvent having thedesired characteristics. Usually for convenience and economy it ispreferable to employ a mixture of solvents, i.e., a solvent andnon-solvent which will result in the desired product.

Suitable solvent mixtures are readily determined for the preparation ofspecific polymer systems by use of the relationship: 6=5 i0.8 wherein 6is the solubility parameter for the solvent system and 6 is thesolubility parameter for the polymer. The solubility parameters arediscussed in Some Factors Affecting the Solubility of Polymers by P. A.Small, Journal of Applied Chemistry 3, 71 (1953) and also by HarryBurnell in the Interchemical Review 14, 3-16, 31-46 (1955). For mixedsolvents the value of 6 is readily calculated by additive averaging on aweight basis. The same technique is also used to determine 6 forcopolymers including those which are highly cross linked, insoluble andnon-swellable. Some typical values for 6 are: polystyrene, 9.1;polydivinyl benzene, 8.8; polymethyl methacrylate, 9.3; polyethylmethacrylate, 9.1; poly n propyl methacrylate, 8.9; poly n butylmethacrylate, 8.7; polymethyl acrylate, 9.7; polyethyl acrylate, 9.2;polybutyl acrylate, 8.7; polyvinyl acetate, 9.4; polyethylenedimethacrylate, 9.2; polyethylene diacrylate, 9.5.

Particularly beneficial and advantageous in the practice of the presentinvention utilizing alkenyl aromatic monomers such as styrene aresolvent or diluent mixtures which have a cohesive energy density of fromabout to 85 and beneficially between about and 75. The cohesive energydensity (c.e.d.) is

wherein AH is the molar heat of vaporization, R is the gas constant, Tis temperature in degrees Kelvin and V is the molar volume (c.e.d.:8Solvents or solvent mixtures having a cohesive energy densityapproaching to when used with alkenyl aromatic resinous polymers such asstyrene-divinylbenzene copolymers give rise to very fine porousnetworks, whereas solvents or solvent mixtures having cohesive energydensities approaching 65 result in relatively coarse porous networks.

In general as the value of 6 approaches that of 6 the pore size of thepolymer decreases and as these values diverge the pore size becomesgreater.

Polymers prepared in according with the method of the present inventionmay be made by mass polymerization of the solvent-polymerizable materialmixture or the mixture may be disposed in the form of droplets in asuitable heat transfer medium.

The present invention is further illustrated, but not limited, by thefollowing examples.

EXAMPLE I Reaction vessels were charged with suitable amounts of styreneand divinyl benzene inert liquid diluents wherein the relationship 6:6:08 was maintained for the majority of the samples and 0.100 percentbenzoyl peroxide based on the total weight of the monomer charged. Thereaction vessels were charged with nitrogen in sufficient quantity toremove at least a major portion of air present, were then sealed andplaced in temperature controlled liquid baths. After a suitablepolymerization period the reaction vessels were opened and the porouspolymer samples removed. Portions of the resulting polymers were takenfor measurement of various physical properties and examination under anelectron microscope. Electron microscope pore size was confirmed bymeans of an Aminco-Winslow mercury porosimeter. The polymer to beevaluated was cut into a suitable section, immersed in methylenechloride for a sufiicient period to remove the original diluenttherefrom and vacuum devolatilized for period of 24 hours at 50centigrade prior to determinations. Pressures up to 2000 pounds persquare inch were utilized. The results are set forth in Table I.

The polymerization schedule used for the samples of Table I was 75centigrade for 24 hours, 85 centigrade for 48 hours, centrigrade for 48hours and centigrade for 48 hours.

Desirable porous polymers are obtained when the solubility parameterfalls within the above range.

TABLE I.-APPEARANCES OF POROUS NETWORK POLYMERS Mole Diluent/ Toluene,Visual Micrograph Pereent Monomer vol. percent Other Diluent AppearanceAppearance 6 Nil Octane Opaque Wh microns 0 0 .do ..do -1 micron .I .0do do 0.l micron 0 Fr ar C MMMMMIQMMNI omwoooo...

Very fine EXAMPLE II In a manner similar to Example I further sampleswere prepared using a polymerization schedule of 70, 80, 90, 100, and120 with 48-hour intervals at each temperature. The relationship resultsare set forth in Table II.

EXAMPLE IV 15 In a manner similar to Example I, a number of samples wereprepared utilizing a polymer schedule of centigrade for 24 hours,centigrade for 48 hours, centi- TABLE II.APPEARANCES OF POROUS NETWORKPOLYMERS Mole percent lliluent/ Toluene, Other Diluent Visual AppearanceMicrograph 6 DVB Monomer vol. percent Appearance 2:1 .1 Translucent 0.1micron 8. 9 $1.0 2:1 Nil Octane..- Opaque wh -10 microns. 7. 6 J. 0 2:1100 Translucent 0.1 micron. 8. 0 8. 0 2: 1 50 Opaque Wh 0.l micron- S. 38. 0 2:1 25 o l. -0.l micron. 8.0 8. 0 2:1 20 -0.l micron. 7. 0 8. 0 2:110 -l micron" 7. 7 8. SI 2:1 Nil 1-3 microns- 7. B 8. 0 0. 5:1 100 (0.1micron. 8.0 8. 0 1:1 100 Translucent... -0.l micron. 8. 0 3. 0 3 :1Opaque Wh -0.l micron. 8. 9 8. 0 0. 5: l d -l micron. 7. 0 8. 9 1:1 1-2microns. 7. 6 8. 0 3:1 -10 microns. 7. 6 8. 0 2:1 0.1%).3 micron 8.9 8.82:1 Nil Octance Opaque wh -1 micorn 7. 6 8. 8

EXAMPLE III In a manner similar to Example I, a plurality of sampleswere prepared using various diluents. The polymerigrade for 48 hours, 90centigrade for 48 hours,

centigrade for 24 hours, and 120 centigrade for 24 hours. The resultsare set forth in Table IV.

TABLE IV.APPEARANCES 0F POROUS NETWORK POLYMERS Monomers Diluents VisualAppearance Micrograph 6 appearance p-Cymerne only 8. 5 8. 0

p"lert butylstyrene and p-Oymene 50%-MeOH 50%.. 11,4 8. 6 DVB. p-Cymene2%-MeOH 75%-. 12.0 8. 6 MeOH only 14.5 8. 6

Ethyl acetate only 9. 1 J. l

Ethyl methacrylate and EtOAe 75%-octane 25%. 8. 8 .I. l ethylene glycoldi- EtOAc 50%-octane 50%. s. 4 .I. l methacrylate. EtOAc 25%-octane 75%.8. 1 0. l Octane only 7. 6 .I. l

I All 06 mole percent monovinyl monomer and 5 mole percent divinylmonomer, with two volumes diluent per volume monomer mixture.

zation schedule was 75 centigrade for 29 hours, 80

centigrade for 40 hours, 85 centigrade for 24 hours, 55 103 centigradefor 48 hours, and centigrade for 48 hours. The results are set forth inTable III.

TABLE III.APPEARANCES OF POROUS NETWORK POLYMERS 1 EXAMPLE V A vesselwas charged with 25 parts of styrene, 8.3 parts of divinyl benzene,0.067 part of benzoyl peroxide, 66.6

1 v1 1 M 60 chromate and 0.2 part of the condensation product of a Diuents sun crograph 6 t Appearance Appearance 1.1 molar mixture ofdlethanol amme and adlpic ac d. The aqueous solution was ad usted to apH of 4.0 with hydrochloric acid. The contents of the vessel were thenPyridine only Opaque tan. 3microns.. 10.9 intimately admixed with highshear agitation until a uniiggg Imgular form dispersion was obtained.The reaction mixture was Pyridie717% Ethylcyclohex- Very fine as thenheated to a temperature of 80 centigrade for a ans Pyridine'%%Ethy10yc10hex 0.1 microns 83 period of 20 hours and gentle agitationmaintained. At ane 91 7 1 3 8 the end of this time the contents werefiltered and the Ethylcyco exane ony 0. microns I. Dionne Ony U 9 70product was mlcroporous beads having an average diame Dioxane Ma-208M71883W 181' Of about 5 l'IllCl'OnS. Dioxanc l7 ectone Dionne M9% Acewm 91 21D The porous polymers of the foregoing examples were Acetone only0.3-1.0 microns.

two volumes diluent per volume monomer mixture, 60 9.1.

10 found to have a surface generally in accordance with their pore sizeand were capable of absorbing relatively large quantities of solutesfrom solutions, and when tightly com- 7 pacted or prepared in the formof a continuous body,

acted very satisfactorily as mieroporous filters for the separation offinely dispersed solids from gases and liquids and also molecularsieves. The porous polymers having pore sizes of about 0.1 micron andsmaller are prepared by maintaining the relationship 6:5 :03.

In a manner similar to the foregoing examples, copolymers of variousacrylate and methacrylate monomers copolymerizable acids, acrylicesters, vinyl copolymers such as vinyl chloride, vinylidene chloride,epoxy resin compositions as well as polyesters are readily prepared inthe form of microporous polymers. Porous polymers hav ing pore sizes ofless than about 0.1 micron are prepared by copolymerizing 90 molepercent of ethyl acrylate with mole percent of diallyl fumarate in thepresence of about 200 volume percent based on the volume of the monomersof chloroform. Somewhat similar beneficial porous copolymers areprepared by copolymerizing 50 parts by weight of acrylonitrile, 50 partsby weight of vinylidene chloride with 10 parts by weight of diethyleneglycol dimethacrylate in the presence of 300 volume percent ethyleneglycol. Generally similar porous polymers are readily prepared bycopolymerizing 70 parts of vinyl chloride, parts vinyl acetate and 10parts of diallyl fumarate in the presence of 250 volume percent ofpropyl acetate. In a similar manner porous polymers may be readilyprepared by reacting such diepoxy compounds as the diglycidyl ether ofBisphenol A with polyhydroxy compounds such as glycerine in the presenceof a solvent wherein 6:6 :03 wherein 5 and 5 are obtained in accordancewith the previously cited references. Also porous polymers are readilyprepared from polyester resins such as are prepared by the condensationof stoichiometric equivalents of phthalic acid and glycerine in thepresence of a solvent where the relationship 6:6 :08 is maintained.

As is apparent from the foregoing specification, the method of thepresent invention is susceptible of being embodied with variousalterations and modifications which may differ particularly from thosethat have been described in the preceding specification and description.For this reason, it is to be fully understood that all of the foregoingis intended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention, excepting as it is set forth and defined in the heretoappended claims.

What is claimed is:

l. The method of preparing a porous polymer having a predeterminedporosity comprising polymerizing (A) polymerizable organic materialwhich is a member selected from the group consisting of [an] (1) alkenylaromatic compounds having the general formula R Ar-o =om wherein Arrepresents an aromatic hydrocarbon radical, or an aromatichalohydrocarbon radical of the benzene series, and R is hydrogen or themethyl radical; (2) acrylate [-type] monomers of the formula cnFo-c onwherein R is selected from the group consisting of hydrogen and an alkylradical containing from about 1 to 12 carbon atoms and R is selectedfrom the group consisting of hydrogen and methyl; [copolymers of vinylchloride and vinylidene chloride, acrylonitrile and vinyl chloride,vinyl bromide] (3) vinyl esters having the formula wherein R is an alkylradical containing from 2 to 18 carbon atoms; [acrylic acid, methacrylicacid, itaconic acid, citraconic acid, maleic acid, fumaric acid, oleicacid, vinylbenzoic acid; the synthetic polyester resins which areprepared by reacting terephthalic acid and dialkyl terephthalicsorester-forming derivatives thereof, with a glycol of the series HO(CH),,OH, wherein n is a whole number within the range of 2-10 and havingreactive olefinic linkages within the polymer molecule, the hereinabovedescribed polyesters which include copolymerized therein up to 20percent by weight of a second acid or ester thereof having reactiveolefinic unsaturation] and (4) mixtures thereof; and (B) at least 10mole percent of a cross-linking agent selected from the group consistingof divinylbenzene, diethylene glycol dimethacrylate, diallyl fumarate,diallyl phthalate and mixtures thereof, to form a rigid cross-linkedpolymer body having a plurality of interconnecting pores therein; saidpolymerization being carried out in [a solvent, the solvent being in aproportion of from] about one-half to about twenty [times the weight]parts by weight of a solvent mixture per part of the polymerizablemixture; [wherein the following relationship exists:] said solventmixture comprising at least one solvent and 10 to 90 percent based onvolume of at least one nonesolvent and having as said mixture asolubility parameter chosen within the range 6:5 :08, where 6 is thesolubility parameter of the solvent and 5 is the solubility parameter ofthe polymer, to control the average pore size of the polymer.

2. The method of claim l wherein the vinyl material is styrene.

3. The method of claim 2, wherein the cross-linking agent isdivinylbenzene.

4. The method of claim 3 wherein the cohesive energy density of thesolvent is from about to about 75.

5. The method of claim 1, wherein the solvent is present in a proportionof from about 1 to 5 times the weight of the polymerizable materials.

6. The method of claim 1, wherein the polymerizable material is preparedin the form of beads by a suspension polymerization technique.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,744,291 5/1956 Stastny et al. 2602.5

2,848,428 8/ 1958 Rubens 2602.5

3,018,257 l/1962 Spencer 2602.5

FOREIGN PATENTS 889,304 2/1962 Great Britain 2602 MURRAY TILLMAN,Primary Examiner M. FOELAK, Assistant Examiner US. Cl. X.R.

